add AddressMode::ClampToZero

[laptou]

Description

On iOS, the "clamp to border" sampler address mode is not supported, however, the "clamp to zero" address mode does seem to be supported, so I would like to use that instead. I have added a new corresponding sampler address mode wgpu::AddressMode::ClampToZero. On the Metal backend I implement this using the native "clamp to zero" address mode, while I implement it using the "clamp to border" address mode on every other backend.

Testing
Test this change by verifying that:

• wgpu::AddressMode::ClampToZero works as expected on all platforms

[laptou]

I could also have implemented this by trying to use ClampToBorder on the Metal backend, and then falling back to ClampToZero if it is not supported and if the border colour is transparent black. However, if the border colour is not transparent black, then it seems like we would have to either ignore it or panic, so I'm not sure that that design would be better.

[kvark]

Thank you for filing this! I didn't know about clamp-to-zero being a separate mode on Metal.
Changes like this should go through https://github.com/gpuweb/gpuweb first, unless you are adding a native-only extension.

There is a problem with your proposal though. Clamp-to-zero returns something that depends on the texture format:

Out-of-range texture coordinates return transparent zero (0,0,0,0) for images with an alpha channel and return opaque zero (0,0,0,1) for images without an alpha channel.

This is different from the clamp-to-border semantics that other backends expose. So it doesn't look like we can implement this portably.

[laptou]

What should I do? I would love to be able to use a clamping address mode on all platforms that I'm targeting (which includes iOS), and if I can't then my shaders will have to become more complicated (and probably slower) because I would have to introduce bounds checks for my texture coordinates.

Would it work to simply document the differences in behaviour (i.e., "On the Metal backend, the sampler behaviour depends on whether the texture has an alpha channel, while on other backends this is equivalent to ClampToBorder with a transparent black border colour"), or is there a workaround/hack that I can use to make this work in my project without merging a change like this into wgpu?

Changes like this should go through gpuweb/gpuweb first, unless you are adding a native-only extension.

Since I am not targeting the web with my application, I'm going to say that this is a native-only extension 😅 someone else can deal with getting this past the W3C if they need it on the web

[kvark]

Ok, let's make it a native-only feature then. We'll have to document the behavior guarantees of Metal, in a similar way you suggested.
It's supported 100% on native, but it has to be a feature because the Web doesn't expose this, and we want applications on wgpu-rs to be targeting the Web seamlessly.

[laptou]

it has to be a feature

What do you mean by this? Do you mean that I should put AddressMode::ClampToZero behind a Cargo feature flag? Or what do I need to do for this change to move forward?

[kvark]

I mean a feature in WebGPU sense, it needs to be a flag in wgt::Features, similar to how clamp-to-border is exposed

[laptou]

Oh, I already implemented that (see here). The feature is available for any adapter where clamp-to-border is available, except for the Metal adapter where it is always available.

[kvark]

Unfortunately, we are hitting another problem here.
I think a solution would require this new behavior to be a part of enum SamplerBorderColor enum (e.g. AutoBlack) instead of the sampler addressing mode enum. This would ensure there is never a conflict between clamping to zero and clamping to other borders.

[laptou]

Okay, I implemented it the way you suggested, but I just ran into a problem with this design when trying to test it on iOS: AddressMode::ClampToBorder is not allowed on iOS with the current settings, so we would need to put in a special case where it checks the border color and panics if we're on iOS and the border colour is not AutoBlack.

[kvark]

Okay, I implemented it the way you suggested, but I just ran into a problem with this design when trying to test it on iOS: AddressMode::ClampToBorder is not allowed on iOS with the current settings, so we would need to put in a special case where it checks the border color and panics if we're on iOS and the border colour is not AutoBlack.

The way validation would work then is the following:

• if ADDRESS_MODE_CLAMP_TO_ZERO is enabled (your new feature), then it's allowed to use border_color = Some(Zero) (or AutoBlack, but I think Zero is closer to the feature name)
• if ADDRESS_MODE_CLAMP_TO_BORDER is enabled, then it's allowed to use border_color = Some(other border colors).
• otherwise, border_color has to be None.

[laptou]

done @kvark

[kvark]

Very nice!

[laptou]

done